Two-component aqueous polyurethane dispersion which comprises an aqueous polyurethane dispersion and a water dispersible crosslinker, such as a water dispersible polyisocyanate or water dispersible isocyanate terminated urethane prepolymer are known.
U.S. Pat. No. 5,334,690 (Schafheutle et al.) discloses sulfonated and/or carboxylated polyurethane dispersions which may be crosslinked with water-emulsifiable prepolymers having terminal isocyanate groups. The reference further discloses that the crosslinking agents are those customarily used in the paint industry.
U.S. Pat. No. 4,574,147 (Meckel) discloses a process for the preparation of polyurethane dispersions wherein polyurethane dispersions containing terminal amine groups are crosslinked with an NCO-terninated polyurethane prepolymer formed from diisocyanates, triols and polyols.
U.S. Pat. No. 5,367,017 (Rosthauser et al.) discloses a process wherein a polyisocyanate is dispersed in water and subsequently reacted with a high molecular weight diamine. A suitable polyamine includes those obtained by hydrolyzing isocyanate-prepolymers having an isocyanate content of 0.5% to 40% by weight.
U.S. Pat. No. 5,523,344 (Maksymkiw et al.) discloses water-based adhesive formulations having enhanced characteristics. As disclosed in the reference, a surprising feature of the preferred embodiments of the invention is that the polyisocyanate crosslinking agent can be added to the adhesive at least a month before use without destabilizing (gelling or high sedimentation) the system and therefore, the crosslinked compositions can be prepared by the supplier before shipping to the end user.
German Utility Model Pat. No. 9304018 discloses an adhesive comprising at least a two component polyurethane dispersion having a pot life of 12 hours or greater. This water-based formulation describes a polymer blend of polyurethane dispersions, EVAs, water dispersible polyfunctional isocyanates and one essentially non-hydrophobic plasticizer.
The references described above disclose two-component aqueous polyurethane dispersions in which a water dispersible polyisocyanate or isocyanate-terminated urethane prepolymer is a crosslinker, and an aqueous polyurethane dispersion is added at, or shortly before, the time it is applied to a substrate and dried. The two-component aqueous polyurethane dispersions have a short pot life (have high settlements or gelled in several hours). The present inventors have found a stable one-component aqueous polyurethane dispersion (have no settlements and have no substantial viscosity change for about six to about twelve months) comprising a reaction product of at least one water dispersible isocyanate-terminated polyurethane prepolymer, at least one aqueous polyurethane dispersion and water.